The Universe is now 1 minute old, and all the anti-matter has been
destroyed by annihilation with matter. The leftover matter is in the
form of electrons, protons and neutrons. As the temperature continues
to drop, protons and neutrons can undergo fusion to form heavier atomic
nuclei. This process is called nucleosynthesis.

Its harder and harder to make nuclei with higher masses. So the most
common substance in the Universe is hydrogen (one proton), followed by
helium, lithium, beryllium and boron (the first elements on the
periodic table).
Isotopes are formed, such as
deuterium and tritium, but these elements are unstable and decay into free
protons and neutrons.

Note that this above diagram refers to the density parameter, , of
baryons, which is close to 0.03. However, much of the Universe is in the form
of dark matter, which brings the value of M to 0.3.

A key point is that the ratio of hydrogen to helium is extremely sensitive
to the density of matter in the Universe (the parameter that determines if
the Universe is open, flat or closed). The higher the density, the more
helium produced during the nucleosynthesis era. The current measurements
indicate that 75% of the mass of the Universe is in the form of hydrogen,
24% in the form of helium and the remaining 1% in the rest of the periodic
table (note that your body is made mostly of these `trace' elements).
Note that since helium is 4 times the mass of hydrogen, the number of
hydrogen atoms is 90% and the number of helium atoms is 9% of the total
number of atoms in the Universe.

There are over 100 naturally occurring elements in the Universe and
classification makes up the periodic table. The very lightest elements
are made in the early Universe. The elements between boron and iron
(atomic number 26) are made in the cores of stars by thermonuclear
fusion, the power source for all stars.

The fusion process produces energy, which keeps the temperature of a
stellar core high to keep the reaction rates high. The fusing of new
elements is balanced by the destruction of nuclei by high energy
gamma-rays. Gamma-rays in a stellar core are capable of disrupting
nuclei, emitting free protons and neutrons. If the reaction rates are
high, then a net flux of energy is produced.

Fusion of elements with atomic numbers (the number of protons) greater
than 26 uses up more energy than is produced by the reaction. Thus,
elements heavier than iron cannot be fuel sources in stars. And,
likewise, elements heavier than iron are not produced in stars, so what is
their origin?.

The construction of elements heavier than Fe (iron) involves nucleosynthesis by
neutron capture. A nuclei can capture or fuse with a neutron because the
neutron is electrically neutral and, therefore, not repulsed like the
proton. In everyday life, free neutrons are rare because they have short
half-life's before they radioactively decay. Each
neutron capture produces an isotope, some are stable, some are
unstable. Unstable isotopes will decay by emitting a positron and a
neutrino to make a new element.

Neutron capture can happen by two methods, the s and r-processes, where
s and r stand for slow and rapid. The s-process happens in the inert
carbon core of a star, the slow capture of neutrons. The s-process
works as long as the decay time for unstable isotopes is longer than the
capture time. Up to the element bismuth (atomic number 83), the
s-process works, but above this point the more massive nuclei that can
be built from bismuth are unstable.

The second process, the r-process, is what is used to produce very
heavy, neutron rich nuclei. Here the capture of neutrons happens in
such a dense environment that the unstable isotopes do not have time
to decay. The high density of neutrons needed is only found during a
supernova explosion and, thus, all the heavy elements in the Universe
(radium, uranium and plutonium) are produced this way. The supernova
explosion also has the side benefit of propelling the new created
elements into space to seed molecular clouds which will form new
stars and solar systems.

Ionization:

The last stage in matter production is when the Universe cools
sufficiently for electrons to combine with the proton/neutron nuclei
and form atoms. Constant impacts by photons knock electrons off of
atoms which is called ionization. Lower temperatures mean photons
with less energy and fewer collisions. Thus, atoms become stable at
about 15 minutes after the Big Bang.

These atoms are now free to bond together to form
simple compounds, molecules, etc. And these are the building blocks for
galaxies and stars.

Radiation/Matter Dominance :

Even after the annihilation of anti-matter and the formation of protons,
neutrons and electrons, the Universe is still a violent and extremely
active environment. The photons created by the matter/anti-matter
annihilation epoch exist in vast numbers and have energies at the x-ray
level.

Radiation, in the form of photons, and matter, in the form of protons,
neutrons and electron, can interact by the process of scattering.
Photons bounce off of elementary particles, much like billiard balls.
The energy of the photons is transfered to the matter particles. The
distance a photon can travel before hitting a matter particle is called
the mean free path.

Since matter and photons were in constant contact, their temperatures
were the same, a process called thermalization. Note also that the
matter can not clump together by gravity. The impacts by photons keep
the matter particles apart and smoothly distributed.

The density and the temperature for the Universe continues to drop as
it expands. At some point about 15 minutes after the Big Bang, the
temperature has dropped to the point where ionization no longer takes
places. Neutral atoms can form, atomic nuclei surrounded by electron
clouds. The number of free particles drops by a large fraction (all the
protons, neutrons and electron form atoms). And suddenly the photons
are free to travel without collisions, this is called decoupling.

The Universe becomes transparent at this point. Before this epoch, a photon
couldn't travel more that a few inches before a collision. So an
observers line-of-sight was only a few inches and the Universe was
opaque, matter and radiation were coupled. This is the transition
from the radiation era to the matter era.

Density Fluctuations:

The time of neutral atom construction is called recombination, this is
also the first epoch we can observe in the Universe. Before
recombination, the Universe was too dense and opaque. After
recombination, photons are free to travel through all of space.
Thus, the limit to our observable Universe is back in time (outward
in space) to the moment of recombination.

The time of recombination is also where the linked behavior between photons
and matter decouples or breaks, and is also the last epoch where
radiation traces the mass density. Photon/matter collisions become rare
and the evolution of the Universe is dominated by the behavior of
matter (i.e. gravity), so this time, and until today, is called the
matter era.

Today, radiation in the form of photons have a very passive
role in the evolution of the Universe. They only serve to illuminate
matter in the far reaches of the Galaxy and other galaxies. Matter, on
the other hand, is free to interact without being jousted by photons.
Matter becomes the organizational element of the Universe, and its
controlling force is gravity.

Notice that as the Universe ages it moves to more stable elements. High
energy radiation (photons) are unstable in their interactions with
matter. But, as matter condenses out of the cooling Universe, a more
stable epoch is entered, one where the slow, gentle force of gravity
dominates over the nuclear forces of earlier times.

Much of the hydrogen that was created at recombination was used up in
the formation of galaxies, and converted into stars. There is very
little reminant hydrogen between galaxies, the so-called intergalactic
medium, except in clusters of galaxies. Clusters of galaxies
frequently have a hot hydrogen gas surrounding the core, this is
leftover gas from the formation of the cluster galaxies that has been
heated by the motions of the cluster members.